The ImageDataBunch creates a validation set randomly each time the code block is run. To maintain a certain degree of reproducibility the np.random.seed() method is built-in within the fastai library.
What Mauro meant by, “random block of the validation set data” was that each time you might want to reproduce your code, ImageDataBunch would automatically choose a random chunk of data from the original dataset. This could be bad or good(you never know 
) according to your use case. Jeremy talks briefly in lesson 2 about it.
So in order to have some control and predictability in WHICH chunk of data should ImageDataBunch create the validation set from, the np.random.seed() is used.